1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to copiers, printers, and like devices which use digital data to produce the hard-copy output image.
2. Description of the Prior Art
Digital halftone processing, which can be used in electronic copiers and printers, offers advantages when reproducing certain types of image data, especially data representing continuous tone images. Some processing techniques associated with halftoning use cluster dot formation in the developed halftone cells. With cluster dot, the individual pixels in the cell grow or develop in an orderly pattern from the center of the cell. When the overall cell density has been determined from pixels which were not located at the center of a cell, the output data does not truly reflect the input data and some of the sharpness of the image is lost in the processing. Thus, the reproduced image is lower in quality than desired.
In an effort to improve the quality of digitally halftoned binary outputs, various systems have been used according to the prior art. Dispersed dot and error diffusion have been used with a certain degree of success. These methods use many single pixel dots in the lower to medium density region, therefore taxing the ability of the reproduction device, or print engine, to maintain consistency throughout the image. Another prior art system functions by looking at the input density within the halftone cell and identifying where teh highest density output pixel is located. The first printed pixel is located at the corresponding location in the output cell. Then, the location of the next highest density output pixel within the cell is determined and the next binary pixel is placed at the corresponding location. The process is continued until the average density represented by the halftone cell is closest to the input cell density. This provides a halftone cell wherein the dots or printed binary pixels vary the location of the center within the cell depending upon the input sharpness and graylevel input information. All of these methods suffer from the fact that they are computationally intensive and require certain restraints is overall system hardware selection and operational speed.
In U.S. Pat. No. 4,246,614, issued on Jan. 20, 1981, a system is described wherein the "black" center of the halftone cell is shifted to align the center-printed pixel with the corresponding "black" center of a video image signal. (See colunm 3, lines 3-14 of that patent.) This process is used in rescreening applications where the original has been screened at one frequency and the digital rescreening is accomplished at another frequency. (See column 1, lines 11-13.) Only the centers of the cells are affected. This patent does not teach a processing system wherein the density distribution of the cell data, as opposed to just the center of the cell, governs the selection of thresholding value patterns for use in the process.
Therefore, it is desirable, and an object of this invention, to provide a digital halftoning system wherein the data is thresholded for binary reproduction without signaificant loss in sharpness detail.